Optical encoder with dual diffraction grating

ABSTRACT

An optical encoder including a light source and a first grating plate having a first diffraction grating for diffracting a light beam emitted from the light source. The optical encoder further includes a second grating plate having a second diffraction grating including a blazed diffraction grating for further diffracting the light beam diffracted by the first diffraction grating so as to allow the light beam to be incident on the first grating plate. The optical encoder also includes a light-receiving portion for receiving the light beam reentering the first grating plate and diffracted by the first grating plate. The second diffraction grating is designed so that the greater part of the diffracted light is concentrated in diffracted light beam of a predetermined order among the light beams from the first diffraction grating, and the diffracted light beam of the predetermined order travels from the second diffraction grating in a direction which is parallel with a direction in which the light beam is incident on the second diffraction grating from the first grating plate. The light-receiving portion generates an electric signal in accordance with an amount of plus and minus mth-order diffracted light beams of the further diffracted light beam.

This is a division of copending application Ser. No. 08/504,708, filedJul. 20, 1995 pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical encoder for convertingmechanical displacement into an electrical signal.

2. Description of the Related Art

With reference to FIG. 13, a conventional exemplary optical encoder willbe described.

FIG. 13 is a schematic diagram showing a conventional optical encoder. Aconventional encoder 100 includes: a light source 101; a collimator lens102; a fixed slit plate 103; a movable slit plate 104; and alight-receiving portion 105. The collimator lens 102 is used forcollimating a light beam emitted from the light source 101. Aftertraveling through the collimator lens 102, the light beam enters theslit plates 103 and 104. The fixed slit plate 103 and the movable slitplate 104 respectively have a plurality of slits which are equal inpitch and parallel to each other. The light-receiving portion 105receives a flux of light passing through the fixed slit plate 103 andthe movable slit plate 104, and converts the flux of light into anelectric signal.

The light source 101, the collimator lens 102, the movable slit plate104 and the light-receiving portion 105 are placed within a movablesection 106. The movable section 106 moves in a direction parallel to aface of the fixed slit plate 103 on which the slits are formed andvertical to the direction of slits.

Hereinafter, the operation of the conventional optical encoder 100having the above-mentioned configuration will be described.

After being collimated by the collimator lens 102, the fixed slit plate103 is irradiated with a light beam emitted from the light source 101.Since the slits on the fixed slit plate 103 and the movable slit plate104 are equal in pitch and parallel to each other, the light beamentering the movable slit plate 104 is transmitted or shielded dependingon the relative position of the slits. Since the movable slit plate 104is placed on the movable section 106 which moves in a direction parallelto the slit of the fixed slit plate 103, the amount of light going outfrom the movable slit plate 104 changes depending on the displacement ofthe movable section 106. The light-receiving portion 105, which isplaced so as to face the side of the movable slit plate 104 from whichthe light beam goes out, converts the change in the amount of light intoan electric signal, thereby detecting the amount of displacement of themovable section 106.

In the conventional optical encoder 100 thus configured, the slit pitchshould be reduced in order to detect the position with higher precision.If the slit pitch is reduced, however, the effect of diffraction isincreased, resulting in the reduced change in the amount of transmittedlight which occurs due to the displacement of the relative positionbetween the slit plates 103 and 104. As a result, difficulty arises indetecting the signal. The effect of diffraction of light can be reducedby shortening the distance between the slit plates 103 and 104. In thiscase, however, there arises a problem that the slit plates 103 and 104are likely to be broken with ease when they are brought into contactwith each other due to impact or vibration. For example, assuming thatresolution of the positional detection is 5 μm per pulse of the signal,the slit pitch of the slit plates 103 and 104 is 5 μm. In this case, itis necessary to set the distance between the slit plates 103 and 104 toseveral μm or less in order to obtain sufficient change in the lightintensity at the light-receiving portion 105.

It is known that when the slits having a pitch p are irradiated withlight using the light source having a small wavelength width and heightcoherence (having a wavelength of λ), the dark and bright patternshaving the same pitch as the slit pitch which are called Fourier images,are generated at the positions expressed by m×(p×p/λ) (m is an integer)in the rear of the slit. The use of these Fourier images makes itpossible to increase the distance between the slits without decreasingthe amount of change in the light intensity at the light-receivingportion 105. In the case where the slit pitch is reduced to 5 μm,however, the distance between the Fourier images is also reduced. Forexample, assuming that the wavelength of the light source is 780 nm, theFourier images are generated at intervals of 32 μm from the slit plate103. If the position of the slit plate 104 deviates from the positionsof the Fourier images due to the change in the distance between the slitplates 103 and 104, the amount of the light intensity at thelight-receiving portion 105 is reduced. Therefore, the variation in thedistance between the slit plates 103 and 104 should be within about 8μm. Moreover, since the odd-numbered Fourier images and theeven-numbered Fourier images have reversed patterns of dark and bright,it is necessary to precisely place the two slit plates 103 and 104 so asto be parallel to each other. In this way, in the conventional opticalencoder 100, it is necessary to precisely adjust the arrangement of thetwo slits 103 and 104.

SUMMARY OF THE INVENTION

The optical encoder of this invention, includes: a light source; a firstgrating plate having a first diffraction grating for diffracting a lightbeam emitted from the light source; a second grating plate having asecond diffraction grating for further diffracting the light beamdiffracted by the first diffraction grating; reflection means forreflecting the light beam from the second grating plate so as to allowthe light beam to reenter the second grating plate; and alight-receiving portion for receiving the light beam reflected by thereflection means and successively diffracted by the second and firstgrating plates, wherein a diffraction angle of plus and minus mth-orderdiffracted light beams of the first diffraction grating is substantiallyequal to that of the plus and minus mth-order diffracted light beams ofthe second diffraction grating, where m is a positive integer, and thelight-receiving portion generates an electric signal in accordance withthe amount of the plus and minus mth-order diffracted light beams of thefirst diffraction grating.

According to one aspect of the invention, the optical encoder includes:a light source; a first grating plate having a first diffraction gratingfor diffracting a light beam emitted from the light source; a secondgrating plate having a second diffraction grating for furtherdiffracting the light beam diffracted by the first diffraction gratingso as to allow the light beam to be incident on the grating plate; and alight-receiving portion for receiving the light beam from the firstgrating plates, wherein a diffraction angle of plus and minus mth-orderdiffracted light beams of the second diffraction grating issubstantially equal to a doubled diffraction angle of the plus and minusmth-order diffracted light beams of the first diffraction grating, wherem is a positive integer, and the light-receiving portion generates anelectric signal in accordance with the amount of the plus and minusmth-order diffracted light beams of the first diffraction grating.

According to another aspect of the invention, an optical encoderincludes: a light source; a first grating plate having a firstdiffraction grating for diffracting a light beam emitted from the lightsource; a second grating plate having a second diffraction grating forfurther diffracting the light beam diffracted by the first diffractiongrating so as to allow the light beam to be incident on the gratingplate; and a light-receiving portion for receiving the light beamreentering the first grating plate and diffracted by the first gratingplates, wherein the second diffraction grating is designed so that thegreater part of the diffracted light is concentrated in a diffractedlight beam of a predetermined order among the light beams from the firstdiffraction grating, and the diffracted light beam of the predeterminedorder travels in a direction in which the light beam is incident on theblazed diffraction grating from the first grating plate, and thelight-receiving portion generates an electric signal in accordance withan amount of plus and minus mth-order diffracted light beams of thefirst diffracted light beams.

An optical encoder of the present invention converts phase change ofplus and minus first order diffracted light beams generated due to thedisplacement of the relative position of a movable plate and a fixedplate into light intensity change by using interference between plus andminus first order diffracted light beams, thereby detecting the obtainedlight intensity change at the light-receiving portion. Thus, even ifgrating pitches of the movable plate and the fixed plates are reduced inorder to enhance the resolution, a signal whose amplitude is neverlowered can be obtained.

Thus, the invention described herein makes possible the advantage ofproviding an optical encoder capable of detecting the position with highprecision and increasing the distance between the plates.

This and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a front view and a side view, respectively,schematically showing the configuration of an optical encoder of Example1 according to the present invention.

FIGS. 2A and 2B are diagrams showing optical paths to a movable plateand a fixed plate in the optical encoder shown in FIGS. 1A and 1B.

FIG. 3 shows an exemplary waveform of an output signal of the opticalencoder shown in FIGS. 1A and 1B.

FIGS. 4A and 4B are a front view and a side view, respectively,schematically showing the configuration of an optical encoder of Example2 according to the present invention.

FIG. 5 is a schematic diagram showing an optical encoder of Example 3according to the present invention.

FIG. 6 is a schematic diagram showing an optical encoder of Example 4according to the present invention.

FIG. 7 is a schematic diagram showing an optical encoder of Example 5according to the present invention.

FIG. 8 is a schematic diagram showing an optical encoder of Example 6according to the present invention.

FIG. 9 is a schematic diagram showing an optical encoder of Example 7according to the present invention.

FIGS. 10A and 10B are diagrams showing optical paths to a movable plateand a fixed plate in the optical encoder shown in FIG. 9.

FIG. 11 is a schematic diagram showing an optical encoder of Example 8according to the present invention.

FIGS. 12A and 12B are diagrams showing optical paths to a movable plateand a fixed plate in the optical encoder shown in FIG. 11.

FIG. 13 is a schematic diagram showing a conventional optical encoder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Hereinafter, an optical encoder 10 of a first example according to thepresent invention will be described by way of illustrative examples.

FIGS. 1A and 1B are schematic diagrams of the optical encoder 10 of afirst example according to the present invention. Specifically, FIG. 1Ais a front view of the optical encoder 10, and FIG. 1B is a side viewthereof. A light source 1 is a semiconductor laser, a light-emittingdiode having a sufficiently small light-emitting portion or the like. Awavelength of the light emitted from the light source 1 is λ. Acollimator lens 2 collimates a light beam emitted from the lightsource 1. A movable plate 3 includes a diffraction grating of arectangular cross-section having a pitch p and a step difference t,which is formed on a transparent substrate having parallel flatsurfaces. The light going out from the collimator lens 2 enters themovable plate 3. A fixed plate 4 also includes a diffraction grating ofa rectangular cross-section having a pitch p and a step difference t,which is formed on a transparent substrate having parallel flatsurfaces. The light going out from the movable plate 3 enters the fixedplate 4. The parallel flat substrates of the movable plate 3 and thefixed plate 4 are made of the same material such as polycarbonate,quartz glass or the like. The step difference t of each of thediffraction gratings of the movable plate 3 and the fixed plate 4satisfies the following Equation 1:

    |n-n.sub.0 |×t=(λ/2)×(1+2j) Equation 1!

where j=0, ±1, ±2 . . . etc., n is a refractive index of a material ofthe movable plate 3 and the fixed plate 4, and n₀ is a refractive indexof a medium between the movable plate 3 and the fixed plate 4. Thedirections of the grooves of the diffraction grating of the movableplate 3 are parallel to those of the fixed plate 4.

A reflective film 5 is formed on the surface of the fixed plate 4opposite to the surface on which the diffraction grating is formed. Thelight reflected from the reflective film 5 is successively rediffractedthrough the fixed plate 4 and the movable plate 3. A light-receivingsection 6 receives the light rediffracted by the fixed plate 4 and themovable plate 3 and the fixed plate 4 so as to output an electric signalin accordance with the amount of received light. The light source 1, thecollimator lens 2, the movable plate 3 and the light-receiving portion 6are placed in a movable section 7 which moves in a direction A-B shownin FIG. 1A. The optical axis of the light going out from the collimatorlens 2 is slightly inclined with respect to a normal direction of thesurface of the fixed plate 4 on which the reflective film 5 is formed,so that the flux of light incident on the reflective film 5 from thelight source 1 can be separated from that reaching the light-receivingportion 6 from the reflective film 5.

Regarding the optical encoder 10 thus configured, the operation thereofwill be described.

First, it will be described that two fluxes respectively diffracted bythe movable plate 3 and the fixed plate 4 overlap each other regardlessof the distance between the movable plate 3 and the fixed plate 4, andthat the interference intensity of the two fluxes of light changesdepending on the displacement of the relative position of the movableplate 3 and the fixed plate 4. Specifically, even if the grating pitchof the diffraction gratings of the movable plate 3 and the fixed plate 4is reduced, a high signal amplitude can be obtained regardless of thedistance between the movable plate 3 and the fixed plate 4, therebyachieving high resolution of the optical encoder.

First, an optical path of the diffracted light in the optical encoder 10will be described. FIG. 2A shows an optical path from the movable plate3 to the reflective film 5 provided for the bottom face of the fixedplate 4, and FIG. 2B shows an optical path from the reflective film 5 tothe movable plate 3.

A light beam emitted from the light source 1 is collimated by thecollimator lens 2 so as to enter the movable plate 3. It is well knownthat the greater part of the amount of light is concentrated in the plusand minus first-order diffracted light beams (about 40% for each) whenthe step difference t of the diffraction grating of the movable plate 3and the fixed plate 4 satisfies the above Equation 1. Therefore, thelight beams going out from the movable plate 3 are a plus first-orderdiffracted light beam 30a and a minus first-order diffracted light beam30b. The fluxes of light 30a and 30b enter the fixed plate 4, and arerespectively diffracted by the diffraction grating of the fixed plate 4.As a result, the fluxes of light going out from the fixed plate 4 becomeplus and minus first-order diffracted light beams. Since the pitch p ofthe movable plate 3 and the fixed plate 4 are equal to each other, thediffraction angles of plus and minus first-order diffracted light beamsare equal to each other. Therefore, as shown in FIG. 2A, a minusfirst-order diffracted light beam 31a of the flux of light 30a and aplus first-order diffracted light 31b of the flux of light 30b arerendered parallel to each other.

Subsequently, after being reflected by the reflective film 5, the fluxesof light 31a and 31b are diffracted by the diffraction grating of thefixed plate 4. The plus first-order diffracted light beam of the lightflux 31a and the minus first-order diffracted light beam of the lightflux 31b become fluxes 32a and 32b, respectively. Then, the light fluxes32a and 32b are diffracted by the movable plate 3, so that the minusfirst-order diffracted light beam of the light flux 32a and the plusfirst-order diffracted light beam of the light flux 32b become fluxes33a and 33b, respectively. Since the diffraction angles of the movableplate 3 and the fixed plate 4 are equal to each other, the light fluxes33a and 33b are parallel to each other and overlap each other withoutbeing affected by the distance between the movable plate 3 and the fixedplate 4.

Next, the change in light intensity due to interference of thediffracted light beams will be described. When the diffraction gratinghaving a grating pitch of p, whose major diffraction light beams areplus and minus first-order diffracted light beams, that is, thediffraction grating is such that the greater part of the diffractedlight is concentrated in plus and minus first-order diffracted lightbeams, displaces in a direction parallel to the grating face andperpendicular to the grooves of the grating by a distance s; the phaseof the diffracted light in a displacement direction of the diffractiongrating and the phase of the diffracted light in a direction opposite tothe displacement direction change by 2πs/p and -2πs/p, respectively.Therefore, when the movable section 7 moves in a direction, for example,a right-hand direction indicated with arrows in FIGS. 2A and 2B, by adistance s, the fluxes of light 31a and 31b are subject to phase changesof 2πs/p and -2πs/p, respectively. Likewise, since the light fluxes 33aand 33b are also subjected to phase changes, the phase changes in thelight fluxes 33a and 33b become 4πs/p and -4πs/p, respectively. Assumingthat the amplitude of the light fluxes 33a and 33b is 1, the lightintensity at the light-receiving portion 6 is obtained as the followingEquation 2.

    |exp(-4πis/p)+exp(4πis/p)|.sup.2 =2{ cos (8πs/p)+1}                                              Equation 2!

where i is an imaginary unit, and i×i=-1.

The above Equation 2 shows that change in relative position of themovable plate 3 and the fixed plate 4 by 1 pitch provides signals of 4pulses. The above mentioned interference of the diffracted light beamsis not affected by the distance between the movable plate 3 and thefixed plate 4. Therefore, since the distance between the movable plate 3and the fixed plate 4 can be increased even when the grating pitch isreduced, it is possible to provide high resolution for the opticalencoder.

For example, in the conventional optical encoder 100 shown in FIG. 13,the pitch can be practically reduced to about 50 μm at most taking thedistance between the slit plates and the like into consideration.Moreover, when the relative position of the slit plates changes by 1pitch, only a signal for 1 pulse can be obtained. On the other hand, inthe optical encoder 10 of Example 1, the pitches of the diffractiongratings of the movable plate 3 and the fixed plate 4 can be reduced toabout p=1 μm, respectively. In this case, the signal for 1 pulse can beobtained with the displacement in the relative position of the movableplate 3 and the fixed plate 4 by 0.25 μm. Therefore, the optical encoder10 of Example 1 may have about 200 times the resolution of theconventional optical encoder 100.

FIG. 3 shows an exemplary waveform of an output signal from thelight-receiving portion 6 of the optical encoder 10 of Example 1. Thediffraction gratings having a pitch p of 10 μm and a step difference tof 0.7 μm are used as the diffraction gratings of the movable plate 3and the fixed plate 4. Quartz glass having a refractive index n=1.45 isused as a material of the movable plate 3 and the fixed plate 4, and anHe-Ne laser having a wavelength λ of 633 nm is used as the lightsource 1. When the relative position of the diffraction gratings of themovable plate 3 and the fixed plate 4 displaces by 2.5 μm, a signal for1 pulse is obtained.

As described above, the optical encoder 10 of Example 1 converts thephase change of the diffracted light due to displacement of the relativeposition between the diffraction gratings into the change in the amountof light due to interference using the movable plate and the fixed platehaving the same pitch and the diffraction gratings whose majordiffracted light beams are plus and minus first-order diffracted lightbeams so as to detect the change in the amount of light. Thus, even whenthe distance between the movable plate and the fixed plate is increased,the amplitude of the signal is not lowered. Therefore, since thedistance between the movable plate and the fixed plate can be setfreely, the movable plate and the fixed plate are prevented from beingdamaged due to contact therebetween and the degree of freedom ofmeasurement can be increased. Moreover, since there is no limitation ofthe distance between the movable plate and the fixed plate when using aFourier image, unlike the conventional optical encoder, fluctuation inthe output of the light-receiving portion with the distance between themovable plate and the fixed plate is narrow. As a result, it is notnecessary to precisely align the two plates.

Since the light source, the light-receiving portion and the movableplate are placed on the movable section, the light-receiving portion ishardly displaced even when the movable section is inclined. Thus, it ispossible to reduce the area of the light-receiving portion so as toimprove a response speed.

The movable and fixed plates are easily duplicated from a stamper.Therefore, it is possible to reduce the cost of the optical encoder.

Although the diffraction gratings of the fixed plate and the movableplate have a rectangular cross-section in Example 1, the diffractiongratings can have a sine wave cross-section or a triangularcross-section.

Although the diffraction grating on the movable plate is provided forthe surface facing the fixed plate, the diffraction grating can beprovided for the surface facing the light source.

In Example 1, the optical axis of the light source is inclined in orderto separate the light beam entering the fixed plate and the light beamgoing out from the fixed plate. In actuality, however, if the opticalaxis of the light source 1 is inclined at an angle in the range of about10° to 20°, the light beams can be sufficiently separated. An opticaldevice for separating light beams can be used instead of inclining theoptical axis. A half mirror can be used as the optical device.Alternatively, the light beam entering the fixed plate and the lightbeam going out from the fixed plate are separated from each other byusing a polarized light beam splitter and a quarter-wave plate. In sucha case, the direction of the optical axis of the quarter-wave plate isat an angle of 45° with the direction in which the maximum amount oftransmitted light is obtained when linearly polarized light enters thebeam splitter.

Although the movable section is displaced and the fixed plate is fixedin Example 1, the fixed plate may be displaced and the movable sectionmay be fixed. Moreover, although the movable plate is fixed to themovable section in Example 1, it is not necessary to fix the movableplate to the movable section so long as the displacement of the relativeposition of the movable plate and the fixed plate can be measured.

Although the reflective film is formed on the bottom face of the fixedplate in Example 1, a reflective mirror may be provided on the movablesection so that the flux going out from the fixed plate is reflectedback to the fixed plate.

EXAMPLE 2

Hereinafter, an optical encoder of Example 2 of the present inventionwill be described with reference to FIGS. 4A and 4B.

Example 2 differs from Example 1 in a method for separating the lightbeam entering the fixed plate and the light beam going out from thefixed plate. In Example 1, the light beam entering the fixed plate andthe light beam going out from the fixed plate are separated from eachother by inclining the optical axis of the light beam entering the fixedplate from the light source with respect to the normal direction of thereflective film. In Example 2, however, the fluxes of light areseparated using a diffraction grating 11. FIG. 4A is a front view of anoptical encoder 20 of Example 2, and FIG. 4B is a side view thereof. Thesame components as those shown in FIGS. 1A and 1B are denoted by thesame reference numerals. A diffraction grating 11 is a transmissiveblazed diffraction grating for concentrating the greater part of thediffracted light in a minus first-order diffracted light beam withrespect to the light emitted from the light source 1 and in a plusfirst-order diffracted light beam with respect to the light going outfrom the fixed plate 4. The diffraction grating 11 is placed so that thegrating face is parallel to the movable plate 3, the fixed plate 4 andthe reflective film 5. The grooves of the diffraction grating 11 areformed perpendicularly to the direction of the grooves of thediffraction gratings of the movable plate 3 and the fixed plate 4.

The operation of the optical encoder 20 of Example 2 is the same as thatof the optical encoder 10 of Example 1 except that the light beamemitted from the light source 1 and the light beam going out from themovable plate 3 respectively enter the movable plate 3 and thelight-receiving portion 6 after being diffracted by the diffractiongrating 11. Specifically, also in the optical encoder 20 of Example 2,the amount of light received by the light-receiving portion 6 changesdepending on the relative position of the movable plate 3 and the fixedplate 4 regardless of the distance between the movable plate 3 and thefixed plate 4. Thus, when the relative position of the movable plate 3and the fixed plate 4 changes by one pitch, the signal of 4 pulses canbe obtained. Therefore, it is possible to enhance the resolution of theoptical encoder 20 of Example 2 as compared with an optical encoder suchas the conventional optical encoder 100 shown in FIG. 13 in which only asignal of 1 pulse is obtained when the relative position of the movableplate 3 and the fixed plate 4 changes by 1 pitch.

Furthermore, since the distance between the movable plate 3 and thefixed plate 4 does not affect the change in the amount of light receivedby the light-receiving portion 6, the distance between the movable plate3 and the fixed plate 4 can be increased. Accordingly, the grating pitchof the diffraction gratings of the movable plate 3 and the fixed plate 4can be respectively reduced. With the reduced pitch, it is possible tofurther enhance the resolution of the optical encoder.

Next, the separation of the light beams entering and going out from thefixed plate 4 by means of the diffraction grating 11 will be described.The light beam emitted from the light source 1 is collimated through thecollimator lens 2 and is incident on the diffraction grating 11perpendicularly to the grating face thereof. The light beam going outfrom the collimator lens 2 is diffracted by the diffraction grating 11.The diffraction grating 11 is a transmissive blazed diffraction gratingwith which the greater part of the diffracted light beam is concentratedin the minus first-order diffracted light beam. Assuming that an angleof diffraction of the diffraction grating 11 is θ₁, the minus firstorder diffracted light beam from the diffraction grating 11 is incidenton the reflective film 5 at an angle of θ₁ with the normal direction ofthe reflective film 5 after being diffracted by the movable plate 3 andthe fixed plate 4. After passing through the fixed plate 4 and themovable plate 3, the light beam reflected by the reflection film 5 isdiffracted by the diffraction grating 11 again. As a result, the minusfirst-order diffracted light beam reaches the light-receiving portion 6.Assuming that the distance from the diffraction grating 11 to thereflection film 5 is α, the light beam entering the fixed plate 4 can beseparated from the light beam going out from the fixed plate 4 by thedistance 2α×tanθ₁. As a result, the light beam going out from the fixedplate 4 can be detected at the light-receiving portion 6.

As described above, according to Example 2, in addition to the enhancedresolution of the optical encoder as in Example 1, the use of thediffraction gratings makes it possible to separate the light beamsentering and going out from the movable plate and the fixed platerespectively without inclining the optical axis of the light beamemitted from the light source.

Also in Example 2, the diffraction gratings can have a sine wavecross-section or a triangular cross-section. Moreover, the diffractiongrating on the movable plate may be provided for the surface facing thediffraction grating for separating the fluxes of light.

Also in Example 2, the movable section or the fixed plate can be movedand it is not necessary to fix the movable plate to the movable sectionso long as the displacement of the relative position of the movableplate and the fixed plate can be measured.

A reflective mirror can be provided on the movable section instead ofproviding the reflective film on the back face of the fixed plate sothat the flux of light going out from the fixed plate enters the fixedplate again.

EXAMPLE 3

Hereinafter, an optical encoder 30 of Example 3 according to the presentinvention will be described with reference to FIG. 5. The configurationof the Example 1 differs from that of Example 3 in the following point.While the diffraction gratings of both the fixed plate and the movableplate are transmissive in Example 1 and the reflective film is providedfor the bottom face of the fixed plate in Example 1, the diffractiongrating of the fixed plate and the diffraction grating of the movableplate are respectively reflective and transmissive and the reflectivefilm is provided on the bottom face of the movable plate. The samecomponents as those shown in FIG. 1 are denoted by the same referencenumerals.

An optical encoder 30 of FIG. 5 has the light source 1, the collimatorlens 2, a movable plate 17, the reflective film 5, a half mirror 8 andthe light-receiving portion 6, which are included in the movable section7, and a fixed plate 18. The half mirror 8 separates a light beamentering the fixed plate 18 from a light beam going out from the fixedplate 18. The fixed plate 18 includes the reflective diffraction gratingof a rectangular cross-section having a pitch p and a step difference(λ/2)×(1+2j)/n₀ of the grating is formed thereon, where j=0, 1, 2 . . .etc., and n₀ is a refractive index of a medium between the fixed plate18 and the movable plate 17. The movable plate 17 includes a diffractiongrating of a rectangular cross-section having a pitch p and a stepdifference t of the grating satisfying Equation 1 described above, whichis formed on one of parallel flat surfaces of the transparent substrate.The reflective film 5 is formed on the face of the movable plate 17,which is opposite to the face facing the fixed plate 18. In the movableplate 17, however, neither diffraction grating nor reflective film 5 isformed on the portion through which the light beam going out from thecollimator lens 2 passes. The size of the portion of the movable plate17 on which the diffraction grating and the reflective film 5 are formedis set taking the diameter of light into consideration.

The operation of the optical encoder 30 having the configuration shownin FIG. 5 will be described. The light beam emitted from the lightsource 1 is collimated by the collimator lens 2, passes though the halfmirror 8, transmits through the movable plate 17, and is diffracted bythe diffraction grating of the fixed plate 18. The diffraction gratingof the fixed plate 18 is formed so that the greater part of thediffracted light is concentrated in plus and minus first-orderdiffracted light beams. Therefore, the plus and minus first-orderdiffracted light beams enter the movable plate 17 as light beams goingout from the fixed plate 18. After being reflected by the reflectivefilm 5, the light beams diffracted by the diffraction grating of themovable plate 17 is diffracted again by the diffraction grating of themovable plate 17. The movable plate 17 is also configured so that majordiffracted light beams can be plus and minus first-order diffractedlight beams. The light beam going out from the movable plate 17 isdiffracted by the diffraction grating of the fixed plate 18 so as toenter the half mirror 8. The half mirror 8 reflects the light beamdiffracted by the fixed plate 18 so that the light beam enters thelight-receiving portion 6. Since the main diffracted light beams of thediffraction grating of the fixed plate 18 are the plus and minusfirst-order diffracted light beams as described above, thelight-receiving portion 6 receives the plus and minus first-orderdiffracted light beams alone.

The change in the interference intensity of the diffracted light beamsdue to displacement of the relative position of the fixed plate 18 andthe movable plate 17 is the same as that in Example 1. Specifically,since the diffraction gratings of the fixed plate 18 and the movableplate 17 are equal in pitch, the interference between two fluxes oflight occurs due to the four-fold diffraction by the fixed plate 18 andthe movable plate 17. Moreover, when the relative position of the fixedplate 18 and the movable plate 17 displaces by s, the phase change of±4πs/p occurs between the diffracted light beams. As a result, thechange in the interference intensity at the light-receiving portion 6 is2{cos(8πs/p)+1}. Therefore, when the relative position of the movableplate 17 and the fixed plate 18 changes by 1 pitch of the grating, asignal for 4 pulses can be obtained. The interference between thediffracted light beams are not affected by change in the distancebetween the movable plate 17 and the fixed plate 18.

As described above, according to Example 3, high resolution can beobtained as compared with the conventional optical encoder 100 withwhich a signal of 1 pulse is obtained for the displacement of therelative position of the movable plate and fixed plate by one pitch.Since the change in the amount of diffracted light beams due tointerference between the diffracted light beams, which is detected atthe light-receiving portion 6, is not affected by the distance betweenthe movable plate 17 and the fixed plate 18, the distance between themovable plate 17 and the fixed plate 18 can be increased even when thegrating pitch is reduced. Thus, it is possible to provide the opticalencoder with higher resolution.

As described above, according to the present example, the diffractiongratings of the fixed plate and the movable plate are reflective andtransmissive, respectively, and the reflective film is provided on theface of the movable plate, which is opposite to the face on which thediffraction grating is formed, thereby obtaining the same effects asthose of Example 1.

Since the light source, the light-receiving portion and the movableplate are placed on the movable section, the light-receiving portion ishardly displaced even when the movable section is inclined. Thus, it ispossible to reduce the area of the light-receiving portion so as toimprove a response speed.

Although the diffraction gratings of the fixed plate and the movableplate have a rectangular cross-section in Example 3, the diffractiongratings can have a sine wave cross-section or a triangularcross-section.

Although the diffraction grating on the fixed plate is provided on thesurface facing the movable plate, the diffraction grating can be formedon the surface opposite to the surface facing the movable plate.

Although the light beam entering the fixed plate and the light beamsgoing out from the fixed plate are separated from each other by usingthe half mirror in Example 3, the separation can be achieved byinclining the optical axis of the light source with respect to thenormal direction of the reflective film instead as in Example 1.Alternatively, a diffraction grating of light can be used as describedabove in Example 2. Alternatively, the light beam entering the fixedplate from the light source and the light beams going out from the fixedplate can be separated from each other using a polarized light beamsplitter and a quarter-wave plate. The direction of the optical axis ofthe quarter-wave plate is at an angle of 45° with the direction in whichthe amount of transmitted light becomes maximum when the linearlypolarized light enters the polarized light beam splitter.

Although the movable section moves and the fixed plate is fixed inExample 3, the fixed plate can be moved and the movable section can befixed instead.

Although the movable plate is fixed to the movable section in Example 3,the movable plate is not necessarily fixed to the movable section. Inshort, as long as the displacement of the relative position between themovable plate and the fixed plate can be measured, it does not matterwhether the movable plate is fixed to the movable section or not.

Although the reflective film is provided for the bottom face of themovable plate, a mirror can be provided on the movable section insteadof the reflective film so that the flux of light returns to the movableplate.

EXAMPLE 4

Hereinafter, an optical encoder 40 of Example 4 according to the presentinvention will be described with reference to FIG. 6. The configurationof the Example 4 differs from that of Example 1 in the following point.While the diffraction gratings of both the fixed plate and the movableplate are transmissive in Example 1 and a reflective film is providedfor the bottom face of the fixed plate in Example 1, the diffractiongrating of the fixed plate and the diffraction grating of the movableplate are transmissive and reflective, respectively. Unlike Examples 1to 3 described above, a reflective film is not used in Example 4 and apitch of the diffraction grating of the movable plate is double that ofthe diffraction grating of the fixed plate. The same components as thoseshown in FIG. 1 are denoted by the same reference numerals.

An optical encoder 40 of FIG. 6 has the light source 1, the collimatorlens 2, a movable plate 19, the half mirror 8 and the light-receivingportion 6, which are all included in the movable section 7, and thefixed plate 4. The fixed plate 4 is a transparent flat substrate similarto that used in Example 1, on one surface of which the diffractiongrating of a rectangular cross-section having a pitch p and a stepdifference t of the grating satisfying the above Equation 1 is formed.The movable plate 19 is a flat substrate having two surfaces parallel toeach other, on one surface of which the reflective diffraction gratingof a rectangular cross-section having a pitch p/2 and a step differenceof (λ/2)×(1+2j)/n₀ is formed, where j=0, 1, 2 . . . etc., and n₀ is arefractive index of a medium between the fixed plate 4 and the movableplate 19. Specifically, both of the diffraction grating of the fixedplate 4 and the diffraction grating of the movable plate 19 are designedso that the main diffracted light beams thereof are plus and minusfirst-order diffracted light beams. In Example 4, the diffractiongrating is formed on the face (surface) of the movable plate 19 facingthe fixed plate 4.

The operation of the optical encoder 40 having the structure shown inFIG. 6 will be described. The light beam emitted from the light source 1is collimated by the collimator lens 2, passes though the half mirror 8,and enters the fixed plate 4. Since the diffraction grating of the fixedplate 4 is formed so that the greater part of the diffracted light isconcentrated in the plus and minus first-order diffracted light beams,the plus and minus first-order diffracted light beams enter the movableplate 19 as light beams going out from the fixed plate 4. The lightbeams going out from the fixed plate 4 are diffracted by the diffractiongrating on the surface of the movable plate 19, and further diffractedby the fixed plate 4. The plus and minus first-order diffracted lightbeams going out from the fixed plate 4 are reflected by the half mirror8, and enter the light-receiving portion 6.

The change in the interference intensity of the diffracted light beamdue to displacement of the relative position of the fixed plate 4 andthe movable plate 19 will be described. Since the pitch of thediffraction grating of the movable plate 19 is half that of thediffraction grating of the fixed plate 4 in Example 4, diffraction angleof the plus and minus first-order diffracted light beams is double thatof the fixed plate 4. Therefore, the light beams diffracted by themovable plate 19 travels in the same direction as that the light beamsentering the movable plate 19 travel in. When the diffracted light beamsof the movable plate 19 enter the fixed plate 4 and are diffracted, theinterference occurs between the two diffracted light beams.

Furthermore, when the relative position of the fixed plate 4 and themovable plate 19 changes by s, the phase change of ±4πs/p occurs betweenthe two diffracted light beams. Therefore, the intensity change of theinterference light beams received at the light-receiving portion 6 is2{cos(8πs/p)+1}. As a result, a signal for 4 pulses can be obtained bythe relative change for 1 pitch of the diffraction grating of the fixedplate 4. Accordingly, the optical encoder 40 has high resolution ascompared with the conventional optical encoder 100 shown in FIG. 13. Theinterference between the diffracted light beams is not affected by thedistance between the movable plate 19 and the fixed plate 4. Therefore,even when the grating pitch is reduced in order to further enhance theresolution, the distance between the movable plate 19 and the fixedplate 4 can be increased. As a result, it is possible to provide higherresolution for the optical encoder 40.

As described above, according to Example 4, the fixed plate has atransmissive diffraction grating, the movable plate has a reflectivediffraction grating, and furthermore, the pitch of the diffractiongrating of the movable plate is half that of the fixed plate. As aresult, the same effects as those of Example 1 can be obtained.

Since the light source, the light-receiving portion and the movableplate are placed on the movable section, the light-receiving portion ishardly displaced even when the movable section is inclined. Thus, it ispossible to reduce the area of the light-receiving portion so as toimprove a response speed.

Although the diffraction gratings of the fixed plate and the movableplate have a rectangular cross-section in Example 3, the diffractiongratings can have a sine wave cross-section or a triangularcross-section.

Although the diffraction grating of the fixed plate is provided on thesurface facing the movable plate in Example 4, it can be provided on thesurface facing the light source.

Although the light beam entering the fixed plate and the light beamsgoing out from the fixed plate are separated from each other by usingthe half mirror in Example 4, the separation can instead be achieved byinclining the optical axis of the light source with respect to thenormal direction of the reflective film. Alternatively, a diffractiongrating of light may be used as described above in Example 2.Alternatively, the light beam entering the fixed plate from the lightsource and the light beams going out from the fixed plate may beseparated from each other using a polarized light beam splitter and aquarter-wave plate. The direction of the optical axis of thequarter-wave plate is at an angle of 45° with the direction in which theamount of transmitted light becomes maximum when the linearly polarizedlight enters the polarized light beam splitter.

Although the movable section moves and the fixed plate is fixed inExample 4, the fixed plate can be moved and the movable section can befixed.

Although the movable plate is fixed to the movable section in Example 4,the movable plate is not necessarily fixed to the movable section. Inother words, the displacement of the relative position of the movableplate and the fixed plate can be measured instead.

EXAMPLE 5

Hereinafter, an optical encoder of Example 5 according to the presentinvention will be described with reference to FIG. 7. Example 1 differsfrom Example 5 in the following point. While the diffraction gratings ofboth fixed plate and movable plate are both transmissive and areflective film is provided for the bottom face of the fixed plate inExample 1, the diffraction gratings of the fixed plate and the movableplate are both reflective and the reflective film is omitted in Example5. Moreover, as in Example 4, the pitch of the diffraction grating ofthe movable plate is double that of the pitch of the fixed plate. Thesame components as those in FIGS. 1A, 1B and 5 are denoted by the samereference numerals.

An optical encoder 50 of FIG. 7 has the light source 1, the collimatorlens 2, a movable plate 20, the half mirror 8 and the light-receivingportion 6, which are all included in the movable section 7, and thefixed plate 18. The fixed plate 18 has a substrate having parallel flatsurfaces; on one of which is formed a reflective diffraction gratinghaving a pitch p and a step difference (λ/2)×(1+2j)/n₀, where j=0, 1, 2,. . . and so forth, so that the greater part of the diffracted light isconcentrated in plus and minus first-order diffracted light beams. Themovable plate 20 also has a substrate having parallel flat surfaces, onone of which a reflective diffraction grating having a step difference(λ/2)×(1+2j)/n₀ is formed. Specifically, the diffraction grating of themovable plate 20 is such that major diffracted light beams are plus andminus first-order diffracted light beams. The pitch of the diffractiongrating of the movable plate 20 is p/2. Moveover, the movable plate 20has an opening through which the light beam going out from thecollimator lens 2 passes. The size of the opening of the movable plate20 is suitably determined taking the diameter of the light beam goingout from the collimator lens 2 and the like.

The operation of the optical encoder 50 having the configuration shownin FIG. 7 will be described. The light beam emitted from the lightsource 1 is collimated by the collimator lens 2, passes though the halfmirror 8 and the opening of the movable plate 20, and is diffracted bythe diffraction grating of the fixed plate 18. Since the majordiffracted light beams of the diffraction grating of the fixed plate 18are plus and minus first-order diffracted light beams, the plus andminus first-order diffracted light beams enter the movable plate 20 aslight beams going out from the fixed plate 18. The plus and minusfirst-order diffracted light beams, which are major diffracted lightbeams among the light beams diffracted by the diffraction grating of themovable plate 20, are diffracted again by the diffraction grating of thefixed plate 18 so as to enter the half mirror 8. The half mirror 8reflects the light beams going out from the fixed plate 18 allowing thelight beams to enter the half mirror 8. As described above, thelight-receiving portion 6 receives the plus and minus first-orderdiffracted light beams from the fixed plate 18, thereby generating anelectric signal in accordance with the change in the light intensity.

The change in the interference intensity of the diffracted light beamdue to displacement of the relative position of the fixed plate 18 andthe movable plate 20 will be described. Since the pitch of thediffraction grating of the movable plate 20 is half that of thediffraction grating of the fixed plate 18 in Example 5, a diffractionangle of the plus and minus first-order diffracted light beams is doublethat of the diffraction grating of the fixed plate 18. Therefore, thelight beams diffracted by the movable plate 20 travels in the samedirection as that the light beams entering the movable plate 20 travelin. When the diffracted light beams of the movable plate 20 enter thefixed plate 18 and are diffracted, interference occurs between the twodiffracted light beams.

Furthermore, when the relative position of the fixed plate 18 and themovable plate 20 changes by s, the phase change of ±4πs/p occurs betweenthe two diffracted light beams. Therefore, the intensity change of theinterference light beams received at the light-receiving portion 6 is2{cos(8πs/p)+1}. As a result, a signal of 4 pulses can be obtained bythe relative change for 1 pitch of the diffraction grating of the fixedplate 18. Accordingly, the optical encoder 50 has high resolution ascompared with the conventional optical encoder 100 shown in FIG. 13. Theinterference between the diffracted light beams is not affected by thedistance between the movable plate 20 and the fixed plate 18. Therefore,even when the grating pitch is reduced in order to further enhance theresolution, the distance between the movable plate 20 and the fixedplate 18 can be increased. As a result, it is possible to provide higherresolution for the optical encoder 50.

As described above, according to Example 5, the fixed plate and themovable plate respectively have reflective diffraction gratings, andfurthermore, the pitch of the diffraction grating of the movable plateis half that of the fixed plate. As a result, the same effects as thoseof Example 1 can be obtained.

Since the light source, the light-receiving portion and the movableplate are placed on the movable section, the light-receiving portion ishardly displaced even when the movable section is inclined. Thus, it ispossible to reduce the area of the light-receiving portion so as toimprove a response speed.

Although the diffraction gratings of the fixed plate and the movableplate have a rectangular cross-section in Example 5, the diffractiongratings can have a sine wave cross-section or a triangularcross-section.

Although both of the diffraction gratings of the movable plate and thefixed plate are both surface reflective in Example 5, at least one ofthem can be bottom face reflective. For example, the diffraction gratingof the fixed plate may be provided on the face opposite to that facingthe movable plate. This arrangement is advantageous in that dust ishardly attached to the diffraction grating face and the like since thediffraction grating of the fixed plate is placed with the diffractiongrating face down.

Although the half mirror is used for separating the light beam enteringthe fixed plate and the light beams going out from the fixed plate inExample 5, the separation can be achieved by inclining the optical axisof the light source with respect to the normal direction of the face ofthe fixed plate on which the diffraction grating is formed as describedin Example 1. Alternatively, the diffraction grating can be used asdescribed in Example 2. Alternatively, the light beam entering the fixedplate from the light source and the light beams going out from the fixedplate can be separated from each other using a polarized light beamsplitter and a quarter-wave plate. The direction of the optical axis ofthe quarter-wave plate is at an angle of 45° with the direction in whichthe amount of transmitted light becomes maximum when the linearlypolarized light enters the polarized light beam splitter.

Although the movable section moves and the fixed plate is fixed inExample 5, the fixed plate may be moved and the movable section can befixed instead.

Although the movable plate is fixed to the movable section in Example 5,the movable plate is not necessarily fixed to the movable section. Inother words, the displacement of the relative position of the movableplate and the fixed plate can be measured instead.

EXAMPLE 6

Hereinafter, an optical encoder 60 of Example 6 of the present inventionwill be described with reference to FIG. 8. Example 6 differs fromExample 1 in the following point. While the diffraction gratings of bothfixed plate and movable plate are transmissive and a reflective film isprovided for the bottom face of the fixed plate in Example 1, thediffraction gratings of the fixed plate and the movable plate arerespectively reflective and transmissive and the reflective film isomitted in Example 6. Moreover, the pitch of the diffraction grating ofthe movable plate is double that of the diffraction grating of the fixedplate. The same components as those in FIGS. 1A, 1B and 5 are denoted bythe same reference numerals.

An optical encoder 60 of FIG. 8 has the light source 1, the collimatorlens 2, a movable plate 12, the half mirror 8 and the light-receivingportion 6, which are all included in the movable section 7, and thefixed plate 18. The movable plate 12 has a transparent substrate havingparallel flat surfaces, on one of which a diffraction grating of arectangular cross-section having a pitch 2p and a step difference tsatisfying the above Equation 1 is formed. Specifically, the diffractiongrating of the movable plate 12 is a transmissive diffraction grating,major diffracted light beams of which are plus and minus firstdiffracted light beams. The fixed plate 18 has a substrate havingparallel flat surfaces, on one of which is formed a reflectivediffraction grating having a pitch p and a step difference(λ/2)×(1+2j)/n₀, where j=0, 1, 2, . . . and so forth. Since the stepdifference is thus set, the major diffracted light beams of thediffraction grating of the fixed plate 18 are plus and minus firstdiffracted light beams.

The operation of the optical encoder 60 having the structure shown inFIG. 8 will be described. The light beam emitted from the light source 1is collimated by the collimator lens 2, passes though the half mirror 8,and is diffracted by the diffraction grating of the movable plate 12.The plus and minus first-order diffracted light beams, which are majordiffracted light beams of the diffraction grating of the movable plate12, enter the fixed plate 18 as light beams going out from the movableplate 12. The fixed plate 18 diffracts the light beams going out fromthe movable plate 12 so that the light beams reenter the movable plate12. The light beams going out from the movable plate 12 are reflected bythe half mirror 8 so as to enter the light-receiving portion 6. In thisway, the light-receiving portion 6 receives the plus and minusfirst-order diffracted light beams from the diffraction grating of themovable plate 12, thereby generating an electric signal in accordancewith the change in light intensity.

The change in the interference intensity of the diffracted light beamdue to displacement of the relative position of the movable plate 12 andthe fixed plate 18 will be described. A signal of 2 pulses is obtainedwith displacement of the diffraction grating of the fixed plate 18 for 1pitch of the grating in Example 6. This is half that of the signalobtained in Example 1. Since the pitch of the diffraction grating of thefixed plate 18 is half that of the diffraction grating of the movableplate 12, the diffraction angle between the plus and minus first-orderdiffracted light beams and the fixed plate 18 is double that of the plusand minus first-order diffracted light beams with respect to the movableplate 12. Therefore, the light beams diffracted by the fixed plate 18travel in the same direction as that of the light beams entering themovable plate 12. Therefore, the interference occurs between the twolight beams diffracted by the movable plate 12. When the relativeposition of the fixed plate 18 and the movable plate 12 changes by s,the phase change of ±2πs/p occurs between the two diffracted lightbeams. Therefore, the intensity change of the interference light beamsreceived at the light-receiving portion 6 is 2{cos(4πs/p)+1}. As aresult, a signal of 2 pulses can be obtained by the relative change for1 pitch of the diffraction grating of the fixed plate 18. Accordingly,the optical encoder 60 has high resolution as compared with theconventional optical encoder 100 shown in FIG. 13 with which only asignal of 1 pulse is obtained by the relative change of the movableplate and the fixed plate for 1 pitch of the slit. The interferencebetween the diffracted light beams is not affected by the distancebetween the movable plate 12 and the fixed plate 18. Therefore, evenwhen the grating pitch is reduced in order to further enhance theresolution, the distance between the movable plate 12 and the fixedplate 18 can be increased. As a result, it is possible to provide higherresolution for the optical encoder 60.

As described above, according to Example 6, the fixed plate and themovable plate respectively have the transmissive diffraction grating andthe reflective diffraction grating, and furthermore, the pitch of thediffraction grating of the movable plate is double that of the fixedplate. As a result, the same effects as those of Example 1 can beobtained.

Since the light source, the light-receiving portion and the movableplate are placed on the movable section, the light-receiving portion ishardly displaced even when the movable section is inclined. Thus, it ispossible to reduce the area of the light-receiving portion so as toimprove a response speed.

Although the diffraction gratings of the fixed plate and the movableplate have a rectangular cross-section in Example 6, the diffractiongratings can have a sine wave cross-section or a triangularcross-section.

Although the diffraction grating on the movable plate is provided on thesurface facing the fixed plate in Example 6, the diffraction grating canbe provided on the surface facing the light source instead.

Although the half mirror is used for separating the light beam enteringthe fixed plate and the light beams going out from the fixed plate inExample 6, the optical axis of the light source can be inclined withrespect to the normal direction of the face of the fixed plate on whichthe diffraction grating is formed as described in Example 1.Alternatively, the diffraction grating of light can be used as describedin Example 2 . Alternatively, the light beam entering the fixed platefrom the light source and the light beam going out from the fixed platecan be separated from each other using a polarized light beam splitterand a quarter-wave plate. The direction of the optical axis of thequarter-wave plate is at an angle of 45° with the direction in which theamount of transmitted light becomes maximum when the linearly polarizedlight enters the polarized light beam splitter.

Although the movable section moves and the fixed plate is fixed inExample 6, the fixed plate can be moved and the movable section can befixed instead.

Although the movable plate is fixed to the movable section in Example 6,the movable plate is not necessarily fixed to the movable section. Inother words, the displacement of the relative position of the movableplate and the fixed plate can be measured instead.

EXAMPLE 7

Hereinafter, an optical encoder 70 of a seventh example according to thepresent invention will be described with reference to FIG. 9. AlthoughExample 7 is similar to Example 1 in that the diffraction gratings ofthe fixed plate and the movable plate are both transmissive, Example 7differs from Example 1 in that the diffraction grating of the movableplate is a blazed diffraction grating. By using the blazed diffractiongrating, an unnecessary diffracted light beam can be prevented, and thelight intensity at the light-receiving portion can be improved. The samecomponents as those in FIGS. 1A, 1B and 5 are denoted by the samereference numerals.

As shown in FIG. 9, an optical encoder 70 has the light source 1, thecollimator lens 2, the fixed plate 4, a movable plate 14, the reflectivefilm 5, the half mirror 8 and the light-receiving portion 6. Thecomponents all but the fixed plate 4 are placed in the movable section7. The fixed plate 4 is the same transparent substrate having parallelflat surfaces as that used in Example 1, on one surface of which thetransmissive diffraction grating of a rectangular cross-section having apitch p and a step difference t satisfying the above Equation 1 isformed. In Example 7, the diffraction grating of the fixed plate 4 isformed on the surface facing the movable plate 14. The movable plate 14is a transparent substrate having parallel flat surfaces, on one surfaceof which the blazed diffraction grating is formed. The blazeddiffraction grating is formed on the bottom face of the movable plate14, that is, the surface of the movable plate 14 opposite to the surfacefacing the fixed plate 4. Moveover, the reflective film 5 is formed onthe surface on which the blazed diffraction grating is formed. The pitchof the blazed diffraction grating is jp/2, where j=1, 2, 3, . . . and soforth. Assuming that a refractive index of the substrate used as themovable plate 14 is n₂, an angle of the grating slant of the blazeddiffraction grating is sin⁻¹ (λ/(n₂ p)). The normal direction of thegrating slant of the diffraction grating is parallel to the direction ofthe plus first-order diffracted light beam or the minus first-orderdiffracted light beam diffracted by the fixed plate 4, respectively. Thedirections of the grooves of the diffraction gratings of the fixed plate4 and the movable plate 14 are parallel to each other. The movablesection 7 moves to a direction which is parallel to the fixed plate 4and perpendicular to the grooves of the blazed diffraction grating ofthe fixed plate 4.

The operation of the optical encoder 70 having the configuration shownin FIG. 9 will be described.

First, it is described that two fluxes of light diffracted by thediffraction gratings of the fixed plate 4 and the movable plate 14overlap each other regardless of the distance between the fixed plate 4and the movable plate 14 and that the interference intensity of the twofluxes changes depending on the relative position of the fixed plate 4and the movable plate 14. Specifically, even when the pitches of thediffraction gratings of the fixed plate 4 and the movable plate 14 arereduced, a high signal amplitude is obtained regardless of the distancebetween the fixed plate 4 and the movable plate 14. Therefore, it ispossible to provide high resolution for the optical encoder.

First, optical paths of the diffracted light beams are described withreference to FIGS. 10A and 10B. FIG. 10A shows an optical path from thefixed plate 4 to the reflective film 5 formed on the bottom face of themovable plate 14, and FIG. 10B shows an optical path from the reflectivefilm 5 to the fixed plate 4.

The light beam emitted from the light source 1 is collimated by thecollimator lens 2, passes through the half mirror 8, and enters thefixed plate 4. Since a step difference t of the diffraction grating ofthe fixed plate 4 satisfies the above Equation 1, the greater part ofthe diffracted light is concentrated in a plus first-order diffractedlight beam 34a and a minus first-order diffracted light beam 34b amongthe light beams diffracted by the diffraction grating of the fixed plate4. The diffraction angle of the diffracted light beams of the fixedplate 4 is θ=sin⁻¹ (λ/(n₀ p)), where n₀ is a reflective index of amedium between the fixed plate 4 and the movable plate 14. The fluxes oflight 34a and 34b are refracted by the surface of the movable plate 14,that is, the surface of the movable plate 14 facing the fixed plate 4 tobe fluxes 35a and 35b.

Assuming that an incident angle of the fluxes of light 35a and 35b onthe blazed diffraction grating is φ, based on Snell's law, sinφ=(n₀/n₂)sinθ=λ/(n₂ p). The pitch of the blazed diffraction grating of themovable plate 14 is jp/2. Therefore, assuming that a diffraction angleof the diffracted light beam is η, 0 sinη={λ/(n₂ p)}×{1-2k/j}, wherek=0, ±1, ±2, ±3 . . . and so forth, and j=1, 2, 3 . . . and so forth.When k =j, the diffraction angle η is equal to -φ and the light beamsgoing out from the blazed diffraction grating travels in the samedirection as that of the light beam incident on the blazed diffractiongrating. The diffraction efficiency of the reflective blazed diffractiongrating depends on the angle of the grating slant, and the greater partof the diffracted light is concentrated in a diffracted light beam whichis a regular reflection with respect to the entering light beam. Anangle in the normal direction of the grating slant of the blazeddiffraction grating of the movable plate 14 is ±sin⁻¹ (λ/(n₂ p)), and isthe same as the azimuth of the fluxes of light 35a and 35b. Thus, thegreater part of the diffracted light is concentrated in diffracted lightbeams 36a and 36b which are reflected in the same direction. Therefore,unnecessary diffracted light beams are not generated at the movableplate 14, thereby improving the optical efficiency.

The fluxes of light 36a and 36b are refracted at the surface of themovable plate 14 as shown in FIG. 10B, enter the fixed plate 4 as fluxesof light 37a and 37b, and are diffracted there. A minus first-orderdiffracted light beam 38a of the flux of light 37a and a plusfirst-order diffracted light beam 38b of the flux 37b are parallel toand overlap each other. The fluxes of light 38a and 38b are reflected bythe half mirror 8, and are directed to the light-receiving portion 6. Inthis way, the light-receiving portion 6 receives the plus and minusfirst-order diffracted light beams 38a and 38b of the fixed plate 4,thereby generating an electric signal in accordance with the change inthe light intensity (that is, the change in the amount of light).

When the relative position of the fixed plate 4 and the movable plate 14changes by s, the phase changes of -4πs/p and +4πs/p occur in the fluxesof light 38a and 38b, respectively. The light intensity at thelight-receiving portion 6 is obtained as expressed by Equation 2 above,assuming that an amplitude of the fluxes of light 38a and 38b is 1.Therefore, it is found that an electric signal of 4 pulses can beobtained with the displacement of the movable section 7 by 1 pitch ofthe diffraction grating of the fixed plate 4. The interference betweenthe diffracted light beams is not affected by the distance between thefixed plate 4 and the movable plate 14. Therefore, even when the gratingpitch is reduced, the distance between the fixed plate 4 and the movableplate 14 can be increased. Thus, it is possible to enhance theresolution of the optical encoder.

As described above, according to Example 7, by using the fixed platehaving the diffraction grating whose major diffracted light beams areplus and minus first-order diffracted light beams and the movable platehaving the reflective blazed diffraction grating, the same effects asthose in Example 1 are obtained. Furthermore, in Example 7, the use ofthe blazed diffraction grating improves the diffraction efficiency atthe movable plate to be about doubled. Thus, an output of the lightsource can be lowered, resulting in the longer life of the light source.

Furthermore, since the light source, the light-receiving portion and themovable plate are placed on the movable section, the position of thefluxes of light on the light-receiving portion is hardly displaced evenwhen the movable section is inclined. Therefore, it is possible toreduce the area of the light-receiving portion so as to improve aresponse speed.

Although the diffraction grating of the fixed plate has a rectangularcross-section in Example 7, the diffraction grating can have a sine wavecross-section or a triangular cross-section.

Although the diffraction grating on the movable plate is provided on thesurface opposite to the surface facing the fixed plate in Example 7, thediffraction grating can be provided on the surface facing the fixedplate instead.

Although the half mirror is used for separating the light beam enteringthe fixed plate and the light beams going out from the fixed plate inExample 7, the optical axis of the light source can be inclined withrespect to the normal direction of the surface of the fixed plate, asdescribed in Example 1. Alternatively, the diffraction grating can beused entering the fixed plate from the light beams going out from thefixed plate as described in Example 2. Alternatively, the light beamentering the fixed plate from the light source and the light beams goingout from the fixed plate can be separated from each other using apolarized light beam splitter and a quarter-wave plate. The direction ofthe optical axis of the quarter-wave plate is at an angle of 45° withthe direction in which the amount of transmitted light becomes maximumwhen the linearly polarized light enters the polarized light beamsplitter.

Although the movable section moves and the fixed plate is fixed inExample 7, the fixed plate may be moved and the movable section can befixed instead.

Although the blazed diffraction grating is formed on the bottom face ofthe movable plate, that is, the surface opposite to the surface facingthe fixed plate in Example 7, the grating can be formed on the surfacefacing the fixed plate. Moreover, a transmissive blazed grating can beformed on the surface facing the fixed plate, and the reflective filmcan be formed on the bottom face thereof instead.

EXAMPLE 8

Hereinafter, an optical encoder 80 of an eighth example according to thepresent invention will be described with reference to FIGS. 11, 12A and12B. In Example 8, the reflective diffraction grating is formed on thesurface of the fixed plate, which is opposite to the surface facing themovable plate, and the reflective blazed diffraction grating is formedon the surface of the movable plate, which is opposite to the surfacefacing the fixed plate. It is possible to prevent unnecessary diffractedlight beams and to increase a signal amplitude at the light-receivingportion by using the blazed diffraction grating. Furthermore, areflective diffraction grating provided for the fixed plate allows themovable section to be compact.

FIG. 11 schematically shows the configuration of an optical encoder 80of Example 8. The light source 1 is a semiconductor laser, alight-emitting diode having a sufficiently small light-emitting portionor the like, which emits a light beams having a wavelength of λ. Thecollimator lens 2 collimates a light beam emitted from the lightsource 1. A fixed plate 22, which receives a collimated light beam fromthe collimator lens 2, has a transparent substrate having parallel flatsurfaces. The diffraction grating of a rectangular cross-section havinga pitch p and a step difference t is formed on the bottom face of thesubstrate, that is, the surface opposite to the surface facing a movableplate 21. The step difference t of the grating is set so as to satisfythe equation: n₁ ×t=(λ/2)×(1+2j), where j=0, 1, 2, 3, . . . and soforth, and n₁ is a refractive index of the material used as the fixedplate 22. Therefore, the greater part of the light beam diffracted bythe diffraction grating of the fixed plate 22 is concentrated in plusand minus diffracted light beams. A reflective film 24 is formed on thesurface of the fixed plate 22 on which grating is formed, as shown inFIG. 11.

The movable plate 21 also has a transparent substrate having parallelflat surfaces, on the top face of which, that is, on the surfaceopposite to the surface facing the fixed plate 22, the blazeddiffraction grating is formed. The pitch of the blazed diffractiongrating is jp/2, where j=1, 2, 3, . . . and so forth. Assuming that arefractive index of the material of the movable plate 21 is n₂, an angleof the grating slant of the blazed diffraction grating is sin⁻¹ (λ/(n₂p)). The normal direction of the grating slant of the blazed diffractiongrating is parallel to the direction of the plus first-order diffractedlight beam or the minus first-order diffracted light beam of the fixedplate 22. The reflective film 23 is formed on the surface of the movableplate 21, on which the blazed diffraction grating is formed. The movableplate 21 has a portion on which the blazed diffraction grating and thereflective film 23 are not formed, through which the light beam goingout from the collimator lens 2 passes. The direction of the grooves ofgrating of the fixed plate 22 is parallel to that of the grooves ofgrating of the movable plate 21.

The half mirror 8 separates a light beam going out from the collimatorlens 2 and light beams going out from the fixed plate 22. The lightbeams going out from the fixed plate 22 are reflected by the half mirror8, and are directed to the light-receiving portion 6 after passingthrough the movable section 21. The light-receiving portion 6 receivesplus and minus first-order diffracted light beams of the diffractiongrating of the fixed plate 22, which go out from the fixed plate 22, soas to convert the change in the amount of light into an electric signal.The light source 1, the collimator lens 2, the movable plate 21, thelight-receiving portion 6 and the half-mirror 8 are placed in themovable section 7. The movable section 7 moves in a direction which isparallel to the fixed plate 22 and perpendicular to the grooves of thediffraction grating of the fixed plate 22.

Next, the operation of the optical encoder 80 of Example 8 will bedescribed.

First, it is described that two fluxes of light diffracted by thediffraction gratings of the movable plate 21 and the fixed plate 22overlap each other regardless of the distance between the movable plate21 and the fixed plate 22 and that the interference intensity betweenthe two fluxes changes depending on the relative position of the movableplate 21 and the fixed plate 22. Specifically, even when the pitches ofthe diffraction gratings of the movable plate 21 and the fixed plate 22are reduced, a high signal amplitude can be obtained regardless of thedistance between the movable plate 21 and the fixed plate 22, therebyallowing the resolution to be enhanced.

First, optical paths of the diffracted light beams to the movable plate21 and the fixed plate 22 of the optical encoder 80 are described withreference to FIGS. 12A and 12B. FIG. 12A shows an optical path from thefixed plate 22 to the reflective film 23 formed on the surface of themovable plate 21 opposite to the surface facing the fixed plate 22, andFIG. 12B shows an optical path from the reflective film 23 to the fixedplate 22.

The light beam emitted from the light source 1 is collimated by thecollimator lens 2, passes through the half mirror 8, and enters thefixed plate 22. The greater part of the diffracted light is concentratedin a plus first-order diffracted light beam 50a and a minus first-orderdiffracted light beam 50b. The diffraction angle θ₁ of the diffractedlight beams of the fixed plate 22 is sin⁻¹ (λ/(n₁ p)), where n₁ is arefractive of a material used as the fixed plate 22. The fluxes of light50a and 50b are reflected when going out from the fixed plate 11 asshown in FIG. 12A, resulting in fluxes 51a and 51b. Assuming that arefractive angle of the fluxes of light 51a and 51b is θ₂, based onSnell's law, n₀ ×sinθ₂ =n₁ ×sinθ₁ is obtained, where n₀ is a refractiveindex of a material between the movable plate 21 and the fixed plate 22.Next, the fluxes 51a and 51b are refracted when entering the movableplate resulting in fluxes 52a and 52b. Assuming that a refractive angleis θ₃, the fluxes 52a and 52b are incident on the blazed diffractiongrating of the movable plate 21 at an angle of θ₃. The incident angle θ₃is obtained as: θ₃ =sin⁻¹ {λ/(n₂ p)}, based on Snell's law.

The fluxes of light 52a and 52b incident on the blazed diffractiongrating at an incident angle θ₃ are diffracted by the blazed diffractiongrating. Assuming that a diffraction angle is η. Since the pitch of theblazed diffraction grating of the movable plate 21 is jp/2, sinη={λ/(n₂p)}×{1-2k/j}, where k=0, ±1, ±2, ±3, . . . and so forth, and j=1, 2, 3,. . . and so forth. Assuming j=k, η=-θ₃ is obtained. Therefore, thelight beams going out from the blazed diffraction grating travels in thesame direction as that of the light beam incident on the blazeddiffraction grating of the movable plate 21. The diffraction efficiencyof the blazed diffraction grating depends on the angle of the gratingslant, and the greater part of the light is concentrated in a diffractedlight beam which is a regular reflection with respect to the enteringlight beam. The normal direction of the grating slant of the blazeddiffraction grating of the movable plate 21 is ±sin⁻¹ (λ/(n₂ p)), and isparallel to the direction in which the flexes of light 52a and 52btravel. Thus, the greater part of the diffracted light is concentratedin diffracted light beams 53a and 53b travelling in the same directionas that of the fluxes of light 52a and 52b. Therefore, unnecessarydiffracted light beams are not generated at the movable plate 21,thereby improving the optical efficiency. As shown in FIG. 12B, thefluxes of light 53a, 54a and 55a reverse the same optical paths as thoseof the fluxes of light 52a, 51a and 50a shown in FIG. 12A. The fluxes oflight 53b, 54b and 55b also reverse the same optical paths as those ofthe fluxes of light 52b, 51b and 50b. Fluxes 55a and 55b are incident onthe diffraction grating of the fixed plate 22, and are individuallydiffracted. A minus first diffraction grating 56a of the light beam 55aand a plus first diffraction grating 56b of the light beam 55b areparallel to each other and overlap each other. After reflected by thehalf mirror 8, the fluxes 56a and 56b are directed to thelight-receiving portion 6 where the change in the amount of light isconverted to an electric signal.

When the movable section 7 moves by the distance s, the relativeposition of the movable plate 21 and the fixed plate 22 changes by s. Asa result, the phase changes of -4πs/p and +4πs/p occur in the fluxes oflight 56a and 56b, respectively. The light intensity at thelight-receiving portion 6 is obtained as expressed by Equation 2 above,assuming that an amplitude of the fluxes of light 56a and 56b is 1.Therefore, it is found that an electric signal of 4 pulses can beobtained with the displacement of the movable section 7 by 1 pitch ofthe diffraction grating of the fixed plate 22. Therefore, according tothe structure of Example 8, it is possible to improve the resolution ofthe optical encoder as compared with the conventional optical encoder100 shown in FIG. 13. Moveover, the interference between the diffractedlight beams has nothing to do with the condition of the distance betweenthe movable plate 21 and the fixed plate 22. Therefore, even when thegrating pitch is reduced, the distance between the movable plate 21 andthe fixed plate 22 can be increased. Thus, it is possible to furtherenhance resolution of the optical encoder.

As described above, according to Example 8, the same effects as those ofExample 1 are obtained by using the fixed plate having the diffractiongrating whose major diffracted light beams are plus and minusfirst-order diffracted light beams and the movable plate having thereflective blazed diffraction grating. Furthermore, in Example 8, sincethe blazed diffraction grating is used, unnecessary light beams are notgenerated. As a result, the diffraction efficiency improved by aboutdouble. Furthermore, by forming a reflective film on the surface of thefixed plate on which the diffraction grating is formed, the movablesection can be compact. Moreover, since the diffraction gratings of themovable plate and the fixed plate are respectively formed on thesurfaces opposite to the surface which receive the light beam, thediffraction efficiency is not lowered even in the case where dust, oiland the like enter the grooves of the diffraction gratings. As a result,a stable output with a large signal amplitude can be obtained. Moreover,since the facing surfaces of the fixed plate and the movable plate areflat, dust, oil or the like can be removed with ease.

Although the diffraction grating of the fixed plate has a rectangularcross-section in Example 8, the diffraction grating can have a sine wavecross-section or a triangular cross-section.

Although the half mirror is used for separating the light beam enteringthe fixed plate and the light beams going out from the fixed plate inExample 8, the optical axis of the light source can be inclined withrespect to the normal direction of the fixed plate. Alternatively, thediffraction grating can be used for separating the light beam enteringthe fixed plate and the light beams going out from the fixed plate.Alternatively, the light beam entering the fixed plate and the lightbeams going out from the fixed plate can be separated from each otherusing a polarized light beam splitter and a quarter-wave plate. Thedirection of the optical axis of the quarter-wave plate is at an angleof 45° with the direction in which the amount of transmitted lightbecomes maximum when the linearly polarized light enters the polarizedlight beam splitter.

Although the movable section moves and the fixed plate is fixed inExample 8, the fixed plate can be moved and the movable section can befixed instead. Neither the fixed plate nor the movable plate isnecessarily fixed as long as the displacement of the relative positionof the fixed plate and the movable plate can be measured.

As described above, in the optical encoder of the present invention, themovable plate and the fixed plate are respectively provided withdiffraction gratings instead of conventional slits. The phase change ofthe diffracted light beams in accordance with the displacement of therelative position of two diffraction gratings is converted into thechange in the amount of light due to the interference between thediffracted light beams. Then, the obtained change in the amount of lightis detected at the light-receiving portion. In this way, with theoptical encoder of the present invention, it is possible to obtain apulse, which is a multiple of the pulse of the optical encoder using aconventional slit plate by an integer, when the relative position of themovable plate and the fixed plate changes by one pitch of the fixedplate. As a result, the position can be detected with higher precision.

The interference between the diffracted light beams is not affected bythe change in the distance between the movable plate and the fixedplate. Therefore, the pitch of the diffraction gratings of the movableplate and the fixed plate is reduced in order to enhance the precision,it is possible to set the distance between the movable plate and thefixed plate so that a signal amplitude is not lowered and the movableplate and the fixed plate are not broken due to contact therebetween andthe like. Moreover, the degree of freedom of measurement can beincreased. Furthermore, unlike the case where Fourier images are used,the distance between the movable plate and the fixed plate is notlimited. Therefore, even when the distance between the movable plate andthe fixed plate is varied, an output at the light-receiving portion ishardly varied.

By using a reflective diffraction grating as at least one of diffractiongratings of the movable plate and the fixed plate, or by usingtransmission diffraction gratings as both of the diffraction gratings ofthe movable plate and the fixed plate and providing a reflective film,the optical path of the light beam diffracted by the movable plate andthe fixed plate is reversed so that the light beam reenters the movableplate and the fixed plate. With this configuration, it is possible torealize the high precision while reducing the optical encoder in size.

The movable and fixed plates are easily duplicated from a stamper.Therefore, it is possible to reduce the fabrication cost of the opticalencoder.

Moreover, when a blazed diffraction grating is used as a diffractiongrating formed on either of the fixed plate and the movable plate, theeffect that the diffraction efficiency at the blazed diffraction gratingis improved to be substantially doubled is also obtained in addition tothe above effects since unnecessary diffracted light beams are notgenerated. As the result of this, the output of the light source can belowered, so that the lifetime of the light source can be prolonged.

Furthermore, since the light source, the light-receiving portion and themovable plate are placed on the movable section, the position of thefluxes of light on the light-receiving portion is hardly displaced.Thus, it is possible to reduce the area of the light-receiving portionso as to improve a response speed.

Furthermore, since the diffraction gratings of the movable plate and thefixed plate are formed on the faces opposite to the faces on which thelight beams are incident, the diffraction efficiency is never loweredeven when dust, oil and the like enter the grooves of the diffractiongratings. As a result, a stable output with a large signal amplitude canbe obtained. Moreover, since the facing surfaces of the fixed plate andthe movable plate are flat, dust, oil or the like can be removed withease.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An optical encoder comprising:a light source; afirst grating plate having a first diffraction grating for diffracting alight beam emitted from the light source; a second grating plate havinga second diffraction grating comprising a blazed diffraction grating forfurther diffracting the light beam diffracted by the first diffractiongrating so as to allow the light beam to be incident on the firstgrating plate; and a light-receiving portion for receiving the lightbeam reentering the first grating plate and diffracted by the firstgrating plate, wherein the second diffraction grating is designed sothat the greater part of the diffracted light is concentrated in adiffracted light beam of a predetermined order among the light beamsfrom the first diffraction grating, and the diffracted light beam of thepredetermined order travels from the second diffraction grating in adirection which is parallel with a direction in which the light beam isincident on the second diffraction grating from the first grating plate,and the light-receiving portion generates an electric signal inaccordance with an amount of plus and minus mth-order diffracted lightbeams of the further diffracted light beam.
 2. An optical encoderaccording to claim 1, wherein the first diffraction grating has agrating step difference which is set so that the plus and minusmth-order diffracted light beams are major diffracted light beams.
 3. Anoptical encoder according to claim 2, wherein the second grating platehas a first surface facing the first grating plate and a second surfacesubstantially parallel to the first surface, and the second diffractiongrating and a reflective film are formed on the second surface.
 4. Anoptical encoder according to claim 3, wherein the first grating platehas a third surface facing the second grating plate and a fourth surfacesubstantially parallel to the third surface, and the first diffractiongrating is formed on the third surface.
 5. An optical encoder accordingto claim 3, wherein the first grating plate has the third surface facingthe second grating plate and the fourth surface substantially parallelto the third surface, and the first diffraction grating and anotherreflective film are formed on the fourth surface.
 6. An optical encoderaccording to claim 5, wherein the second grating plate has a portion onwhich neither the second diffraction grating nor the reflective film isformed, and the light beams emitted from the light source passes throughthe portion so as to enter the first grating plate.
 7. An opticalencoder according to claim 2, further comprising means for separatingfluxes of light provided between the light source and the first gratingplate.
 8. An optical encoder according to claim 7, wherein the means forseparating fluxes of light is a half mirror.